Pacing therapy can be used in the treatment of heart failure, which refers to a clinical syndrome in which an abnormality of cardiac function causes a below normal cardiac output that can fall below a level adequate to meet the metabolic demand of peripheral tissues. When uncompensated, it usually presents as congestive heart failure due to the accompanying venous and pulmonary congestion. It has been shown that some heart failure patients suffer from intraventricular and/or interventricular conduction defects (e.g., bundle branch blocks) such that their cardiac outputs can be increased by improving the synchronization of ventricular contractions with electrical stimulation. Cardiac rhythm management devices have therefore been developed which provide electrical stimulation to the ventricles in an attempt to improve the coordination of cardiac contractions, termed cardiac resynchronization therapy (CRT).
The degree to which a heart muscle fiber is stretched before it contracts is termed the preload, while the degree of tension or stress on a heart muscle fiber as it contracts is termed the afterload. When a myocardial region contracts late relative to other regions, the contraction of those other regions stretches the later contracting region and increases its preloading, thus causing an increase in the contractile force generated by the region. Because pressure within the ventricles rises rapidly from a diastolic to a systolic value as blood is pumped out into the aorta and pulmonary arteries, the parts of the ventricles that contract later during systole do so against a higher afterload than do parts of the ventricles contracting earlier. Thus, a ventricular region that contracts later than other regions is subjected to both an increased preload and afterload, both of which act to increase the mechanical stress experienced by the region relative to other regions.
Resynchronization pacing may be delivered in a manner that pre-excites one or more hypertrophied regions in order to subject the regions to a lessened preload and afterload. For example, the ventricles may be paced at multiple sites using a multi-site resynchronization pacing mode, where the delivery of paces to multiple ventricular sites during a cardiac cycle is used to not only enforce a minimum ventricular heart rate, but also to alter the depolarization patterns of the ventricles during systole and improve the coordination of the ventricular contraction. The pulse output sequence can be specified so that one or more hypertrophied regions are paced before other regions during systole and hence mechanically unloaded. By unloading such hypertrophied regions in this way over a period of time, reversal of undesirable ventricular remodeling is effected.
Implantable medical devices have been developed that provide appropriately timed electrical stimulation to one or more heart chambers in an attempt to improve the coordination of ventricular contractions during CRT. Ventricular resynchronization is useful in treating heart failure because resynchronization results in a more coordinated contraction of the ventricles with improved pumping efficiency and increased cardiac output. Currently, a common form of CRT applies stimulation pulses to both ventricles, either simultaneously or separated by a specified biventricular offset interval, and after a specified atrio-ventricular delay interval with respect to the detection of an intrinsic atrial contraction.
In summary, current optimization strategies in cardiac pacing aim to ensure an optimal loading of the ventricles (e.g., atrio-ventricular delay optimization). Also in CRT, an inter-ventricular optimization can lead to an improved cardiac performance as measured by left ventricular pressure gradient.